1. Field of the Invention
The present invention generally relates to a neural interface and, more specifically, to a transcutaneous neural interface.
2. Description of the Prior Art
Muscle paralysis affects over one hundred thousand people in the United States and approximately one million people worldwide. One class of patients who face severe difficulties in their daily lives is those with locked-in syndrome. Locked-in syndrome patients generally have a cognitively intact brain and a nearly completely paralyzed body. They are alert but cannot move or talk. They face a life-long challenge to communicate. Some patients may use eye movements, blinks or remnants of muscle movements to indicate binary signals, such as “yes” or “no.” One approach used to provide assistance to patients with locked-in syndrome has been described in U.S. Pat. No. 4,852,573, which is hereby incorporated by reference (see, e.g., col. 5, 11. 39 et seq. for a discussion of use of nerve growth factor to enhance nerve growth into an electrode). In this approach, an electrode is implanted into the patient's brain and signals from the electrode may be used to control an electronic device.
Neural interfaces may be implanted in human subjects for communication and motor restoration applications. An ideal invasive neural interface would include multiple recording sites, isolation of each recording site to one axon to avoid cross-talk, mechanical stability, biocompatibility, long term recording capabilities, and a size minimization.
The ability to record neural activity over long durations is critically dependent upon proper neural interface design. Current approaches involve silicon microelectronic machining techniques for controllability and size reduction of passive electrodes. The desire for multiple recording sites has led to the development of multi-electrode arrays with corresponding high-density micro-ribbons. However these devices have not been able to achieve long term recording due to signal degradation and artifacts due to gliosis and micro-movements of the components.
One type of electrode recording system that has been available for long term human implantation is the neurotrophic electrode, which includes hollow glass cone containing gold recording wires that allow recording from axons grown into the glass cone under the influence of neural trophic factors. Recording from this reconstituted neuropil has produced action potentials (APs) that display robust signal-to-noise ratios over long time periods. The recording system uses transcutaneous FM transmission of the amplified system, thereby avoiding the need for wires. It is powered by air gap induction coils, obviating the need for batteries. This system has been implanted in six locked-in humans to provide them with control of a switch or a computer cursor, thus restoring synthetic speech, Internet access, environmental control, and other applications.
However, existing applications have implanted electrodes in only one or a few sites. This limited number of implanted sites limits the amount of information that can be transmitted to the external interface.
An increase in the number of electrode sites and size reduction of the recording area for individual axon isolation could substantially increase the likelihood of success in such applications as speech synthesis. The current neurotrophic electrode has a cone diameter of 20-25 μm and can contain anywhere between 10-50 individual neurites. Reducing the diameter into the 1-5 μm range would effectively limit the number of neurites grown into the device. The increase of neural data obtained from multiple recording sites would require additional wiring from the device to the amplifier system located outside of the skull. By applying a wireless method of transmitting signals from the neural interface to the electronics mounted on the skull, a high throughput of data can be achieved without the introduction of bulky micro-ribbons.
Direct optical imaging of neural activity has been demonstrated through voltage sensitive or Ca 2+-sensitive dyes with a charge coupled device (CCD) camera for detection. The CCD camera system allows simultaneously recording of multiple neural activities over a surface area at up to 5 kHz resolution. However, as the entire cortical surface area is bathed with the dye, there is no selectivity over the neurons being observed. The CCD camera is used to monitor electrical activity of all neurons over a 2-dimension visual field. This approach is not appropriate for long term recordings because of bleaching and phototoxic effects of the dyes.
Quantum dots have received substantial attention for biological marking applications utilizing photoluminescence, where higher energy light induces a characteristic (lower energy) quantum dot light emission. The electric dipole created at the quantum dot during optical light adsorption may be large enough to stimulate or inhibit neuronal firing. Quantum dots offer an advantage of increased efficiency. This is attributed to the inverse square relationship between charge and distance. By using either antibody-antigen recognition or peptide recognition groups, the quantum dots may be bound to the neuron and separated by less than 10 nanometers, as opposed to micron ranges in current neural interfaces.
The long term stability of such an approach has yet to be seen. Quantum dots (QDs) are mostly formed from Cd based heterostructures and although they are used routinely in vivo for marking applications, there are still existing questions as to their toxicity. Organic capping layers have been created to envelop the quantum dots, however these layers tend to act as electrical traps that increase efficiency for optical labeling applications, but reduce efficiency of opto-electrical conversion. In addition, quantum dots do not form long term interfaces, typically only lasting a matter of weeks. Tethered quantum dot films have been studied to address the question of stability but these films degraded within 3-5 days in primary neuron cultures. While cell binding techniques allow precise cell selection, they do not provide a method that can control interface construction at specified site areas.
Improvements to high density neural electrodes would substantially increase neural signal throughput and usability. Most notably, an increase in the number of electrode sites and size reduction of the recording area for individual axon isolation are critical requirements. Existing neurotrophic electrodes have a cone diameter of 20-25 μm and can contain anywhere between 10-50 individual neurites. Reducing the diameter into the 1-2 μm range would effectively limit the number of neurites grown into the device. The increase of neural data obtained from multiple recording sites using existing electrodes would require additional wiring from the device to the amplifier system located outside of the skull. By applying a wire-free method of transmitting signals from the neural interface to outside the skull, a high throughput of data can be achieved without the introduction of bulky cables.
Therefore, there is a need for a stable neural interface that transmits local neural action potentials from a plurality of recording sites.